Page 27 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com certain sugars. For example, Neisseria gonorrhoeae and mebooksfree.com
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16
PART I Basic Bacteriology
chemicals such as sugars, amino acids, and salts, such as
Neisseria meningitidis can be distinguished from each other
sodium chloride. Often blood is added in the form of
on the basis of fermentation of either glucose or maltose
sheep’s blood but that is for nutritional purposes rather
(see page 131), and E. coli can be differentiated from Salmo-
than for the need of the bacteria to grow within the red
nella and Shigella on the basis of fermentation of lactose
blood cells.
However, certain bacterial pathogens of humans, nota-
bly Chlamydia and Rickettsia (see Chapters 25 and 26,
The term fermentation refers to the breakdown of a
respectively), and Ehrlichia and Anaplasma (see Chapter
sugar (such as glucose or maltose) to pyruvic acid and then,
usually, to lactic acid. (More specifically, it is the break-
26) can only grow within living cells. They are obligate (see page 151).
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intracellular parasites, meaning that it is obligatory that
down of a monosaccharide such as glucose, maltose, or
mebooksfree.com mebooksfree.com mebooksfree.com β-galactosidase in E. coli before fermentation can occur.) mebooksfree.com
galactose. Note that lactose is a disaccharide composed of
they grow within cells. The main reason for this is that they
glucose and galactose and therefore must be cleaved by
lack the ability to produce sufficient ATP and must use
ATP produced by the host cells.
Fermentation is also called the glycolytic (glyco = sugar,
lytic = breakdown) cycle, and this is the process by which
AEROBIC & ANAEROBIC GROWTH
facultative bacteria generate ATP in the absence of
For most organisms, an adequate supply of oxygen enhances
oxygen.
metabolism and growth. The oxygen acts as the hydrogen
If oxygen is present, the pyruvate produced by fermen-
acceptor in the final steps of energy production catalyzed
tation enters the Krebs cycle (oxidation cycle, tricarboxylic
by the flavoproteins and cytochromes. Because the use of
2
oxygen generates two toxic molecules, hydrogen peroxide
and H O. The Krebs cycle generates much more ATP than
2
–
(H O ) and the free radical superoxide (O ) bacteria
the glycolytic cycle; therefore, facultative bacteria grow
2
2
2
require two enzymes to detoxify these molecules when acid cycle) and is metabolized to two final products, CO
faster in the presence of oxygen. Facultative and anaerobic
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mebooksfree.com mebooksfree.com - 2 2 + mebooksfree.com aeruginosa, produce metabolites that enter the Krebs cycle mebooksfree.com
oxygen is utilized. The first is superoxide dismutase,
bacteria ferment, but aerobes, which can grow only in the
which catalyzes the reaction
presence of oxygen, do not. Aerobes, such as Pseudomonas
2O + 2H → H O + O
2
2
2
2
by processes other than fermentation, such as the deamina-
and the second is catalase, which catalyzes the reaction
tion of amino acids.
In fermentation tests performed in the clinical labora-
2H O → 2H O + O
tory, the production of pyruvate and lactate turns the
2
2
medium acid, which can be detected by a pH indicator that
The response to oxygen is an important criterion for
changes color upon changes in pH. For example, if a sugar
classifying bacteria and has great practical significance
because specimens from patients must be incubated in a
the pH becomes acidic and the medium turns yellow. If,
proper atmosphere for the bacteria to grow.
however, the sugar is not fermented, no acid is produced
(1) Some bacteria, such as M. tuberculosis, are obligate is fermented in the presence of phenol red (an indicator),
and the phenol red remains red.
mebooksfree.com mebooksfree.com mebooksfree.com Iron, in the form of ferric ion, is required for the growth mebooksfree.com
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aerobes; that is, they require oxygen to grow because their
ATP-generating system is dependent on oxygen as the
IRON METABOLISM
hydrogen acceptor.
(2) Other bacteria, such as E. coli, are facultative anaer-
obes; they utilize oxygen, if it is present, to generate energy
of bacteria because it is an essential component of cyto-
by respiration, but they can use the fermentation pathway
chromes and other enzymes. The amount of iron available
to synthesize ATP in the absence of sufficient oxygen.
for pathogenic bacteria in the human body is very low
(3) The third group of bacteria consists of the obligate
because the iron is sequestered in iron-binding proteins
anaerobes, such as Clostridium tetani, which cannot grow
such as transferrin. To obtain iron for their growth, bacte-
in the presence of oxygen because they lack either superox-
ide dismutase or catalase, or both. Obligate anaerobes vary
phores. Siderophores, such as enterobactin produced by
in their response to oxygen exposure; some can survive but
E. coli, are secreted by the bacteria, capture iron by chelat-
are not able to grow, whereas others are killed rapidly. ria produce iron-binding compounds called sidero-
ing it, then attach to specific receptors on the bacterial
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surface, and are actively transported into the cell where
the iron becomes available for use. The fact that bacteria
FERMENTATION OF SUGARS
ing iron testifies to its importance in the growth and
In the clinical laboratory, identification of several impor-
metabolism of bacteria.
tant human pathogens is based on the fermentation of
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